The growth and healing ability of the skeleton is possible because postnatal skeletal stem cells (SSCs) contin- ually generate bone-forming osteoblasts. Platelet-derived growth factor receptor ? (PDGFR?) is expressed on SSCs and osteoblasts, but its functional roles have not been characterized in vivo. Recently, humans with gain-of-function mutations in PDGFRB have been reported to exhibit skeletal disease involving progressive bone loss or skeletal overgrowth during childhood or adolescence. However, the target cell type and molecular mechanisms underlying PDGFR?-driven skeletal disease are unknown. The applicant?s long-term goal is to develop a mechanistic understanding of how the PDGF pathway regulates mesenchymal cell plasticity. Bone forms because osteoblasts produce collagen-rich organic matrix called osteoid, which subsequently becomes mineralized into bone. Defects in cell plasticity and collagen production may underlie bone diseases driven by the PDGF pathway. Therefore, the specific objective in this proposal is to identify PDGF-regulated mecha- nisms controlling SSCs and their contribution to skeletal disease. The hypothesis underlying this project is that PDGFR? regulates SSC proliferation and differentiation through the balance of downstream effectors of the signal transducer and activator of transcription (STAT) family. Aim 1 will use an SSC-targeted Cre/lox ap- proach to induce PDGFR? activating mutations and combinatorial STAT deletions, and determine whether STATs mediate bone disease in vivo. Primary SSCs derived from mutant mouse bones will be used to investi- gate whether PDGFR?-regulated STAT signaling regulates SSC self-renewal and differentiation in vitro or after transplantation. As an alternative approach, PDGFR? activating mutations will be targeted to chondrocytes. Aim 2 will characterize new mouse models with the PDGFR? activating mutations most commonly found in Penttinen syndrome and Kosaki overgrowth syndrome, V665A and P584R, respectively. Aim 2 will also ex- plore the benefits of kinase inhibitors for mice with gain-of-function PDGFR? signaling. Aim 3 will study mice with gain- and loss-of-function mutations in PDGFR? to identify the processes by which PDGFR? regulates SSCs and their progeny during postnatal skeleton growth. Mutant cells will be fate mapped to determine how different levels of PDGFR? activity regulate proliferation, differentiation, and cell fate in vivo. This work is ex- pected to define how the PDGFR? signaling pathway mediates osteogenesis and how too much or too little signaling generates diseases of the skeleton, which will point to novel therapeutic strategies and better ap- proaches for bone repair. The results of these projects will significantly advance understanding of SSC regula- tory mechanisms. Information about signaling pathways and cell types that mediate disease-associated PDG- FR? signaling will inform the development of new therapeutic approaches for skeletal diseases. And identifying the specific role of PDGFR? in bone growth will begin to establish it as a molecular target for therapy. All of this information will improve the restoration of structure and function to diseased or damaged bones.